This invention relates to optical fiber communication cables adapted to be used indoors or outdoors.
Optical fiber cables are used for transmitting information between various devices including telephone exchanges, computers, etc. The optical fibers are small in diameter, and are relatively fragile. Therefore, they must be protected from mechanical stresses encountered during manufacture, installation and operation such as bending and pulling stresses. One known way of protecting optical fibers is to place them loosely in tubes with the fibers longer than the tubes and to associate with such tube a tensile member, e.g. a stranded metal wire, a high strength plastic or carbon, glass or graphite fibers, which absorbs the tensile stresses. The tubes with the tensile member are enclosed in a sheath or sheaths. See, for example, U.S. Pat. Nos. 4,078,853; 4,153,332; and 4,230,395.
With the optical fiber loosely received in the tubes, there are unfilled spaces within the tubes, and the fibers are usually protected from moisture and other hydrogen compounds. It is known to fill such tubes with a grease-like material which prevents migration of such moisture and compounds in the tubes but which permits relative movement of the fibers within their enclosing tubes. See, for example, U.S. Pat. Nos. 4,230,395; 4,722,589 and 4,725,121.
When the tubes containing the fibers are of circular or other cross-section and are enclosed by a sheath of circular or other cross-section, there are spaces between the tubes, and it is desirable to prevent the ingress of moisture into the latter spaces. It is known to fill such latter spaces with a grease-like material similar to, or the same as, the grease-like material used to fill the tubes, or hydrogen absorbing compound, but which permits relative movement of the tubes. See, for example, U.S. Pat. No. 4,230,395.
Such tubes containing the optical fibers are often wound helically around a central supporting structure which can contain the tensile member. In some cases, they are placed on the central supporting structure in what is called an S and Z shaped configuration, or in other words, they are wound around the central supporting structure in a first direction, or hand, for one or several turns and then, the direction of winding is reversed for one or several turns. Such reversal is continued periodically. See, for example, U.S. Pat. Nos. 4,697,875; 4,722,589 and 4,725,121.
The foregoing description is typical of optical fiber cables used outside of buildings and a typical diameter of optical fiber with a primary coating used in outdoor cables is 250 um. However, optical fiber cables used inside a building have different requirements and different structures. Thus, "indoor" cable have a tight buffer coating, which is necessary to meet the mechanical performance of an indoor cable, have a typical buffer coating diameter of 900 um. Also other coating diameters have been used in the range of 250 to 1200 um. The cable must meet the requirements of the National Electrical Code, e.g. Article 770, with respect to fire or flame resistance and low smoke producing characteristics, whereas outdoor cables are not required to meet such requirements.
Accordingly, problems arise when an outdoor cable must be connected to an indoor cable, i.e. due to the different cable structures causing termination and splicing problems. While some optical fiber cables meet some of the requirements for cables to be used either indoors or outdoors, such cables do not meet all the requirements for both uses. For example, they do not meet the water penetration test used for an outdoor cable and/or light buffered optical fibers do not meet the tensile loading requirements for an outdoor cable. Also, filled outdoor cables do not meet the fire tests applicable to indoor cables.